| Does it suck? #225671 12/19/10 07:38 PM 12/19/10 07:38 PM | MN3
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Unregistered | I read somewhere that "low pressure actually sucks you forward, you are not being pushed upwind... but being sucked"
is this true? how can it be? feel free to get as technical as you wish.... | | | Re: Does it suck?
[Re: hobie1616]
#225677 12/20/10 05:33 AM 12/20/10 05:33 AM |
Joined: Jun 2001 Posts: 9,582 North-West Europe Wouter
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Posts: 9,582 North-West Europe | It is true.
The suction forces on your sails are larger then the overpressure forces and therefore you more "sucked" then "pushed" forward.
Wouter
Wouter Hijink Formula 16 NED 243 (one-off; homebuild) The Netherlands
| | | Re: Does it suck?
[Re: Wouter]
#225678 12/20/10 06:18 AM 12/20/10 06:18 AM |
Joined: Jun 2009 Posts: 51 Richmond, Va soccerguy83
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journeyman
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Posts: 51 Richmond, Va | Same concept as an airplane wing air has to move faster over the curved leward side of your sail than the windward side causing a low pressure on the leward side.
Brian C. H14 H16
| | | Re: Does it suck?
[Re: soccerguy83]
#225680 12/20/10 06:58 AM 12/20/10 06:58 AM |
Joined: Jan 2005 Posts: 6,049 Sebring, Florida. Timbo
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Posts: 6,049 Sebring, Florida. | From Wikipedia:
Real-world application Condensation visible over the upper surface of a wing caused by a pressure drop due to air accelerationIn modern everyday life there are many observations that can be successfully explained by application of Bernoulli's principle, even though no real fluid is entirely inviscid [19] and a small viscosity often has a large effect on the flow.
Bernoulli's Principle can be used to calculate the lift force on an airfoil if you know the behavior of the fluid flow in the vicinity of the foil. For example, if the air flowing past the top surface of an aircraft wing is moving faster than the air flowing past the bottom surface then Bernoulli's principle implies that the pressure on the surfaces of the wing will be lower above than below. This pressure difference results in an upwards lift force.[nb 1][20] Whenever the distribution of speed past the top and bottom surfaces of a wing is known, the lift forces can be calculated (to a good approximation) using Bernoulli's equations[21] – established by Bernoulli over a century before the first man-made wings were used for the purpose of flight. Bernoulli's principle does not explain why the air flows faster past the top of the wing and slower past the underside. To understand why, it is helpful to understand circulation, the Kutta condition, and the Kutta–Joukowski theorem.
Blade F16 #777
| | | Re: Does it suck?
[Re: Timbo]
#225681 12/20/10 07:04 AM 12/20/10 07:04 AM |
Joined: Jan 2005 Posts: 6,049 Sebring, Florida. Timbo
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Posts: 6,049 Sebring, Florida. | So...in our world, ie. racing sailboats, if you are going upwind, you are being Sucked. If you are going downwind, you are being Blown.
Sucked or Blown, that is the question. And here's some light math for you to figure it out:
value of effort The plots of air strike sail on both sides so:
On the windward side efforts are atmospheric pressure, wind pressure, and virtually no depression due to wind. On the leeward side efforts are atmospheric pressure, a bit of depression and almost no wind pressure. To simplify the manipulation of these forces, the forces are summed into a single force and that the entire surface of the profile (sailing) in a simple formula (valid for airplane wings like a rudder, a sail, an anti Plan -drift) [7]:
[8]
with
E = effort that can give up the wind (see Max Q); C = coefficient Aerodynamic According to the Bernoulli, the maximum stress of wind or density of kinetic energy maximum for the entire surface of the sail:
The full expression of the force is:
with
F = lift, expressed in Newton ρ (rho) = density air (ρ varies with the temperature and the pressure) ; S = typical surface, for sail, it is the sail area in m² C = coefficient Aerodynamic. Aerodynamic coefficient is unit-less, it is the sum of two percentages: the percentage of recovered energy leeward side + percentage of the recovered energy into the wind. For this reason, the coefficient aerodynamics can be greater than 1. It depends on the angle of upwind sailing. V = Speed is the speed of the wind relative to the sail (Apparent wind) in m / s. The sail is deformed by the wind and takes a form named Airfoil. When the flow of air around the profile is Laminar [9], the factor against depression in the wind becomes crucial. This effect is then called lift. Studies and theory to draw a sail [10] that:
Depression on the upper (leeward side) represents two thirds of the lift, The pressure on the lower surface (facing the wind) represents one third of the lift. [edit] Lift effect on sail The study effect of lift can compare cases with and without lift [11]. A typical example is a gaff sail. The sail is rectangular and is approximately vertical. The sail has an area of 10 sqm, with 2.5m of foot by 4m of leech. The apparent wind is 8.3 m / s (about 30 km / h). The boat is supposed to uniform velocity, no wave. It does not heel, does not pitch. The density of air is set at: ρ = 1.2kg / m3
[edit] turbulent flow or downwind The boat is running downwind. The shape of the sail is approximated by a plane perpendicular to the apparent wind.
The depression effect on the sail is second order, and therefore negligible, it remains:
On the windward side, efforts are atmospheric pressure and wind pressure On the leeward side, there remains only the atmospheric pressure Efforts to atmospheric pressure cancel out. There remains only pressure generated by the wind.
Roughly speaking, shock of parcel on the sail forward all their energy from wind in 90% of the surface of the sail. This means that the Cz or aerodynamic lift coefficient is equal to 0.9.
[edit] laminar flow The boat is Close hauled. The wind has an angle of about 15 degrees with chord of the sail.
Because the setting of the sail at 15 ° relative to the apparent wind, the camber of the sail creates a lift. In other words, the effect of depression on the leeward side is not neglected. As air pressure forces cancel out, efforts remain are:
On the windward side, wind pressure, On the leeward side, wind depression. The only unknown is the drag coefficient to be estimated. Curve takes a good adjustment of sail is close to upper shape NACA 0012[12][13]. A sail less well adjusted or older technology (old rig), will be more hollow, more camber. The coefficient of aerodynamic lift will be higher but the sail will be less efficient (lower finesse). The profiles would be more suitable profiles as NACA 0015, NACA 0018 [14].
For a given profile, there are tables which giving the lift coefficient of the profile. The lift coefficient (Cz) depends on several variables:
Incidence (angle: apparent wind / Profile) The lift hill of the sail, which depends on its extension, The surface roughness and Reynolds number, which affect the flow of fluid (laminar, turbulent). The coefficient is determined for a fluid stable and uniform, and a profile of infinite extension.
The Reynolds number is:
with
U - fluid velocity or apparent wind [m / s] L- characteristic length or foot of the sail [m] ν - kinematic viscosity fluid: ν = η / ρ [m / s] ρ - density air [kg / m³] μ - dynamic viscosity air [Pa] or Poiseuille [pl] so for this sail about Re = 106
Under an incidence of 15 ° and a Reynolds number to one million, reached a NACA0012 Profile Cz 1.5 instead of 0.9 for 90 ° incidence.
The lift has increased by 50%. This also corresponds on sheet an increase of 50% effort for the same apparent wind.[15][16]
Blade F16 #777
| | | Re: Does it suck?
[Re: Timbo]
#225682 12/20/10 07:17 AM 12/20/10 07:17 AM |
Joined: Jan 2005 Posts: 6,049 Sebring, Florida. Timbo
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Posts: 6,049 Sebring, Florida. | Aww crap, the formulas didn't copy/paste for some reason. Well, you get the general idea.
Blade F16 #777
| | | Re: Does it suck?
[Re: Timbo]
#225683 12/20/10 07:21 AM 12/20/10 07:21 AM |
Joined: Jan 2004 Posts: 1,884 Detroit, MI mbounds
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Pooh-Bah
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Posts: 1,884 Detroit, MI | where L is lift force, ρ is air density v is true airspeed, A is planform area, and CL is the lift coefficient at the desired angle of attack, Mach number, and Reynolds number Also, where: L is the lift, A is the wing surface area p is the value of the pressure, n is the normal unit vector pointing into the wing, and k is the vertical unit vector, normal to the freestream direction. | | | Re: Does it suck?
[Re: Tony_F18]
#225686 12/20/10 07:34 AM 12/20/10 07:34 AM |
Joined: Jan 2005 Posts: 6,049 Sebring, Florida. Timbo
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Posts: 6,049 Sebring, Florida. | Yes, be very affraid, and drive to your next destination.
We don't build them, we just fly them. Pull back, the houses get smaller, push foreward, the houses get bigger.
That's about all there is to it. Just ask those Quantas boys on that A380 with the exploding engine...
Blade F16 #777
| | | Re: Does it suck?
[Re: Timbo]
#225687 12/20/10 07:41 AM 12/20/10 07:41 AM |
Joined: Jan 2009 Posts: 5,525 pgp
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Posts: 5,525 | rofl! Clearly you need more time on the water.
Pete Pollard Blade 702
'When you have a lot of things to do, it's best to get your nap out of the way first.
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[Re: pgp]
#225688 12/20/10 07:47 AM 12/20/10 07:47 AM |
Joined: Oct 2006 Posts: 119 Bernhards Bay, N.Y. popeyez7
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Posts: 119 Bernhards Bay, N.Y. | ~~ I don't care if I'am gettin "BLOWN" or "SUCKED" on my boat...... If it's "WET, It's "FUN" ~~~~ ;)~
~~Don't fear the ''SPEED''~~ Fear the''ADDICTION'' ~17,18 Hobies, ~Jet boat~Speedster 150 ~2 Kayaks~ ~~~~VIETNAM VET. 69-71~~~~
| | | Re: Does it suck?
[Re: Tony_F18]
#225690 12/20/10 07:53 AM 12/20/10 07:53 AM |
Joined: Jan 2005 Posts: 6,049 Sebring, Florida. Timbo
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Posts: 6,049 Sebring, Florida. | You might want to ask the people who got stuck in the Chunnel first. And that's only a short tunnel (26mi?) under the English Channel.
I was sitting in a pub in Brighton the day the Chunnel opened many years ago, watching all the hoopla on the TV over the bar. There was a reporter asking the Britts on site what they thought about now being connected by road directly to France.
One guy said, "Well I don't think it's a good idea." The reporter asked him, "Why, don't you like France?" He said, "I LOVE France, it's the French I hate!"
Classic!
Blade F16 #777
| | | Re: Does it suck?
[Re: Timbo]
#225692 12/20/10 08:06 AM 12/20/10 08:06 AM |
Joined: Jan 2007 Posts: 160 North Carolina abbman
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Posts: 160 North Carolina | I think the term "suck" is misleading. How can an air molecule pull on anything? It's like the old egg in a bottle experiment. The egg is pushed into the bottle, not sucked or pulled in.
James 1983 Hobie 16'
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[Re: abbman]
#225693 12/20/10 08:13 AM 12/20/10 08:13 AM |
Joined: Jan 2005 Posts: 6,049 Sebring, Florida. Timbo
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Posts: 6,049 Sebring, Florida. | Define Suction.
Using your logic, the term should not exist.
In training they told us a jet engine works like this:
Suck, Squeeze, Bang, Blow. I think a recip works the same way, but with more moving parts...
Blade F16 #777
| | | Re: Does it suck?
[Re: abbman]
#225694 12/20/10 08:23 AM 12/20/10 08:23 AM |
Joined: Dec 2001 Posts: 5,590 Naples, FL waterbug_wpb
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Posts: 5,590 Naples, FL | Tim, I take issue with the "no substance is invicid". Look up Bose-Einstein Condensate. This material has ZERO viscosity
Even better, scientists have actually used this super-cold substance to slow the speed of light to roughly the speed of your average bicycle (without being diffracted). Now you can actually say you're moving "faster than light" (under certain conditions, of course)
Jay
| | | Re: Does it suck?
[Re: waterbug_wpb]
#225695 12/20/10 08:35 AM 12/20/10 08:35 AM |
Joined: Jan 2009 Posts: 5,525 pgp
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Posts: 5,525 | So I should rub it on my sails?
Pete Pollard Blade 702
'When you have a lot of things to do, it's best to get your nap out of the way first.
| | | Re: Does it suck?
[Re: pgp]
#225696 12/20/10 08:46 AM 12/20/10 08:46 AM |
Joined: Dec 2001 Posts: 5,590 Naples, FL waterbug_wpb
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Posts: 5,590 Naples, FL | interesting thought. maybe wear gloves It would be hard to time your finish, however, with all that quantum flux going on... But your finish time could be sooner than your start time... How do you handicap that?
Jay
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